Synthesis and topology analysis of chloridotriphenyl(triphenyl phosphate-κO)tin(IV)

In the title compound, a (3,−1) critical point is found on the topology path connecting the (PhO)3P=O and SnPh3Cl moieties, showing that an actual O—Sn covalent bond is formed between the phosphate and the stannane derivatives.

The title Sn IV complex, [Sn(C 6 H 5 ) 3 Cl(C 18 H 15 O 4 P)], is a formal adduct between triphenyl phosphate (PhO) 3 P O and the stannane derivative chloridotriphenyltin, SnPh 3 Cl. The structure refinement reveals that this molecule displays the largest Sn-O bond length for compounds including the X O!SnPh 3 Cl fragment (X = P, S, C, or V), 2.6644 (17) Å . However, an AIM topology analysis based on the wavefunction calculated from the refined X-ray structure shows the presence of a bond critical point (3,À1), lying on the interbasin surface separating the coordinated phosphate O atom and the Sn atom. This study thus shows that an actual polar covalent bond is formed between (PhO) 3 P O and SnPh 3 Cl moieties.

Chemical context
An interesting feature of tin(IV) is its ability to perform as a hypervalent centre: pentacoordinated tin compounds, like chlorido(dimethyl sulfoxide)triphenyltin, SnPh 3 (DMSO)Cl (Pouye et al., 2018), are as common as tetracoordinated tin compounds, for example chloridotriphenyltin, SnPh 3 Cl (Tse et al., 1986;Ng, 1995). This leaves the possibility open to synthesize compounds with intermediate valency, between four and five. The title compound is such a compound, which is formally obtained as the adduct of SnPh 3 Cl and triphenylphosphate, (PhO) 3 P O, for which the X-ray structure is available (Svetich & Caughlan, 1965). While the phosphate group P O coordinates the Sn centre, more than four electrons in the valence shell of Sn, 4d 10 5s 2 5p 2 , must be involved in the formation of the bonds around Sn. Herein, we are interested in the nature of the bond between Sn and the phosphate O atom.

Structural commentary
The title molecule, SnPh 3 Cl-(PhO) 3 P O, crystallizes in space group P1 with one molecule in the asymmetric unit (Fig. 1). The P O group of the phosphate coordinates the Sn centre, trans to the Cl atom, with a P-O-Sn angle of 177.58 (12) . The five-coordinate Sn centre displays a distorted trigonalbipyramidal geometry, very different from the tetrahedral geometry observed for SnPh 3 Cl, and consistent with dsp 3 hybrid orbitals on the metal centre. Conversely, the phosphate moiety in the title compound features a tetrahedral geometry close to that of free (PhO) 3 P O. The main structural feature is the staggered arrangement of the six phenyl rings, minimizing intramolecular steric hindrance. The same conformation was previously obtained in the adduct between SnPh 3 Cl and triphenylphosphine oxide Ph 3 P O (Ng & Kumar Das, 1992) or in the complex chlorido[chloromethyl(diphenyl)phosphine oxide]triphenyltin, SnPh 3 Cl-Ph 2 (CH 2 Cl)P O (Kapoor et al., 2007).
In the title compound, the Sn-O bond length is 2.6644 (17) Å . A survey of the CSD shows that for X O!SnPh 3 Cl fragments where X = P, S, C or V, the X O-Sn angles range from 119.4 to 176.3 , while Sn-O bond lengths range from 2.29 to 2.64 Å (CSD 5.43 with all updates; Groom et al., 2016). There is no correlation between the bond lengths and angles (R 2 = 0.002 for a linear fit). The largest Sn-O bond in the set of 40 structures retrieved from the CSD is 2.642 Å , for a dinuclear Sn complex (Gholivand et al., 2015) closely related to the title compound. The title complex has thus the largest Sn-O bond length and P O-Sn angle in this series, which could reflect a bond order less than 1 for the bond Sn-O. The situation is quite different, for example, for a non-hindered phosphastanninane, which forms dimers through P O-Sn bonds, with a short Sn-O bond length of 2.425 Å (Weichmann & Meunier-Piret, 1993). However, in the title compound, the SnPh 3 Cl moiety is certainly bound to the phosphate, since the sum of van der Waals radii for Sn and O is 3.69 Å , much larger than the observed Sn-O separation (Bondi, 1964). In other words, SnPh 3 Cl-(PhO) 3 P O can not be described as a co-crystal between SnPh 3 Cl and (PhO) 3 P O. This can be confirmed through the topology analysis of electron density in the complex, and in particular the computation of critical points, in the context of the Bader's QTAIM theory (quantum theory of atoms in molecules; Bader, 2009). Therefore, starting from the SHELXL refinement (Table 1), a wave function was calculated using ORCA (Neese, 2018), and the structural model further refined with olex2.refine and NoSpherA2 (Bourhis et al., 2015;Kleemiss et al., 2021) within OLEX2 (Dolomanov et al., 2009). The relativistic basis set x2c-SVP and the generalized gradient approximation PBE functional were used. This refined model included isotropic H atoms with free coordinates, and converged to R 1 = 3.26%, a slight improvement over the SHELXL refinement at R 1 = 3.48%.
A (3,À1) bond critical point is then observed at the midpoint of the atomic pair O1/Sn1, lying on the interbasin surface separating atoms O1 and Sn1 (Fig. 2). The charge density for this critical point is = 0.024 a.u. (corresponding to 2.552 Â 10 10 C m À3 ), and a topology bond path connects the    nuclear critical points (3,À3) placed on O1 and Sn1. The nature of the Sn1-O1 bond can be further characterized by computing the Laplacian of the electron density, r 2 ð Þ, in the vicinity of the bond: in the valence-atomic orbital region between the O and Sn atoms, the bond critical point has a small critical density and a positive Laplacian (Fig. 3). Regions combining ! 0 and r 2 ð Þ > 0 are dominated by closedshell interactions suffering from Pauli repulsions, as in ionic bonds (for an extremely clear and well-written introduction to the valence-bond theory in the AIM context, see Shaik et al., 2015). In the present case, the Sn1-O1 bond can thus be seen as a polar single (covalent) bond mainly characterized by electrostatic interactions. This description is obviously consistent with the large electronegativity gap between Sn and O, Á % 1:5 on the Pauling scale. Moreover, the bond polarization is reflected in calculated CHELPG charges (atomic charges fitting the molecular electrostatic potential; Breneman & Wiberg, 1990): +0.597 for Sn1 and À0.543 for O1, as calculated by Multiwfn (Lu & Chen, 2012).

Supramolecular features
Although six phenyl rings are present in the molecular complex, its conformation does not favour the emergence of interactions in the crystal structure. The only relevant intermolecular interactions are weak C-HÁ Á ÁO contacts. Two neighbouring complexes are connected through weak interactions between the oxygen atoms O3 in the (PhO) 3 P O moieties, and the hydrogen atoms H30A belonging to neighbouring molecules (d HÁ Á ÁO = 2.71 Å and C-HÁ Á ÁO = 146.8 ; Table 2, entry 1). These interactions lead to discrete dimers, forming centrosymmetric R 2 2 (8) ring motifs (Fig. 4). Other similar contacts in the crystal have their C-HÁ Á ÁO angles below 120 (Table 2, entry 2), and are thus expected to have no contribution to crystal stabilization (Wood et al., 2009 Table 2 Hydrogen-bond geometry (Å , ). Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 2.

Figure 2
Contour map of the electron density (brown contour lines) with the gradient vector field of (green flux lines) in the vicinity of the P O-Sn-Cl group. Bond and nuclear critical points are represented by blue and brown dots, respectively, while the purple bold lines are the bond paths (Bader, 2009) connecting nuclear critical points. The map was calculated and plotted using Multiwfn (Lu & Chen, 2012).  Dimeric cluster in the crystal structure, formed through weak C-HÁ Á ÁO hydrogen bonds (dashed blue lines).
The mixture was refluxed (T = 473 K) under stirring for 1 h. The obtained solution was slightly cloudy, then it was filtered off. The filtrate was slowly evaporated at 300 K for one week, to give colourless crystals suitable for X-ray diffraction.

Refinement details
Crystal data, data collection and structure refinement details are summarized in  (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).